A carbon nanotube has a tubular shape that one face of graphite is wound, one wound in one layer is called a single-walled carbon nanotube, one wound in two layers is called a double-walled carbon nanotube and one wound in multilayer is called a multi-walled carbon nanotube. Since a carbon nanotube has high mechanical strength and high electrical conductivity, it has been expected as an anode material for fuel cells and lithium secondary batteries, and as materials composed of composite materials with resin, metal and organic semiconductors for high strength resins, conductive resins, transparent conductive films, metal electrolytic powders, ceramic complexes and electromagnetic shielding materials. Further, since a carbon nanotube has a large L/D (length/outer diameter ratio) and an outer diameter of several nm, it has been expected as materials for a scanning tunnel microscope probe, field electron emission source and nano-pincette. Further, since a carbon nanotube has a nanosized space, it has been expected as adsorption materials of hydrogen etc., as materials of nanocapsule for medical purpose and MRI contrast agent. In any application, a carbon nanotube of high purity is required, and a single-walled or double-walled carbon nanotube with a small outer diameter is advantageous. Further, a carbon nanotube with few defects in graphite layer is excellent in characteristics.
As a method for producing carbon nanotubes, an arc discharge method, laser evaporation method, chemical vapor deposition method and the like are known. Above all, as a method for producing carbon nanotubes with few defects in graphite layer and high quality inexpensively, a catalyst chemical vapor deposition method is known. In the catalyst chemical vapor deposition method, there is known a method where a catalyst is supported on a support.
Non-patent document 1 reports a synthesis method of double-walled CNT using magnesia having mesoporous pores as a support, methane as a carbon source, and cobalt as a metal. However, as a reactor, a horizontal type is used, thus it is difficult to synthesize a lot of carbon nanotubes efficiently.
The present inventors have found that a method of contacting a catalyst that a metal is supported on a support with a carbon-containing compound at high temperature is a method for obtaining carbon nanotubes of high quality at high yield. Further, conventionally, carbon nanotubes are synthesized by disposing a catalyst in a horizontal reactor, but the present inventors have found that by using a vertical reactor, a raw gas can be contacted uniformly with a catalyst to react efficiently (Patent document 1). Synthesis of carbon nanotubes using a vertical reactor is also disclosed in Patent document 2.    Non-patent document 1: Chemical Physics Letters 368 (2003), 299-306    Patent document 1: Japanese Unexamined Patent Publication No. 2004-123505    Patent document 2: Tokuhyo Application No. 2006-511437 (Japanese translation of PCT publication)